Lactococcus petauri, a gram-positive coccus described in 2017, is a novel species closely related to Lactococcus garvieae [1]. Owing to their nearly identical 16S rRNA sequences and phenotypic similarities, discrimination of the two species is difficult using conventional methods [1, 2]. L. garvieae, a well-known fish pathogen, has been recognized as a zoonotic agent associated with endocarditis and spondylitis [3, 4]. L. petauri infections are well-documented in aquaculture; however, reports in humans are limited. To date, only one human case of urinary tract infection has been reported [5]. A report described the genome sequence of a bloodstream isolate, but without clinical or susceptibility data [6]. To our knowledge, we report the first well-documented human case of L. petauri bloodstream infection with concurrent infective endocarditis and spondylitis. The Institutional Review Board of Seoul National University Hospital, Seoul, Korea, approved this study (2205-142-1327) and waived the need for informed consent due to the study’s retrospective nature.

In May 2022, an 88-yr-old woman with hypertrophic cardiomyopathy and gastrostomy was referred to the emergency department for anemia and a 2-month history of back pain, during an outpatient clinic visit. Fig. 1A summarizes the clinical timeline. At presentation, Hb was 71 g/L (reference interval [RI]: 120–160 g/L); leukocyte count, 8.99×10⁹/L (RI: 4–10×10⁹/L); neutrophil ratio, 84.1%; C-reactive protein, 127.7 mg/L (RI: 0–5 mg/L); and NT-proBNP, 2,133 pg/mL (RI: 0–200 pg/mL). Spine magnetic resonance imaging revealed a paravertebral abscess. Two sets of blood samples were collected from peripheral veins, inoculated into BACTEC Plus Aerobic/F and Anaerobic/F bottles (Becton Dickinson, Sparks, MD, USA), and incubated in a BACTEC FX system. After 6 hr, two aerobic bottles were positive for gram-positive cocci. Subculture on blood agar yielded smooth, circular white colonies.

The isolate could not be identified using the Gram-Positive Identification card of the VITEK 2 system (bioMérieux, Durham, NC, USA). Subsequent matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS; MALDI Biotyper; Bruker Daltonics, Bremen, Germany) identified the isolate as L. garvieae (score: 1.95), consistent with genus-level identification. 16S rRNA gene sequencing and GenBank Basic Local Alignment Search Tool analysis indicated 100% identity with both L. garvieae (LC376029.1) and L. petauri (CP045924.1); gyrB sequencing showed the same limitation. The isolate was resistant to trimethoprim–sulfamethoxazole and susceptible to benzylpenicillin, vancomycin, levofloxacin, meropenem, ceftriaxone, and erythromycin (Table 1).

Echocardiography revealed vegetation on the anterior mitral leaflet, fulfilling the modified Duke criteria for definite infective endocarditis. The patient received ceftriaxone, which was later changed to oral amoxicillin/clavulanate for planned discharge; however, intermittent fever delayed discharge. All follow-up blood cultures tested negative. Despite temporary improvement, she developed respiratory failure with worsening mitral regurgitation and died on day 71 from acute pulmonary edema due to valve rupture.

Whole-genome sequencing (WGS) of the isolate was performed using the MiSeq platform (Illumina, San Diego, CA, USA). Reads were assembled de novo using Unicycler v0.5.0, yielding a genome of 1,972,844 bp (GC content, 38.15%) in 52 contigs (N50, 223,722 bp). Average nucleotide identity (ANI) analysis against National Center for Biotechnology Information reference genomes showed 98.1% similarity to L. petauri, exceeding the 95%–96% ANI threshold for species-level identification, and 93.8% similarity to L. garvieae. Phylogenetic analysis supported this finding (Fig. 1B).

Human infections with L. petauri are rare and may have been underrecognized due to misidentification as L. garvieae. In a recent study on fish isolates, 60.4% of strains previously classified as L. garvieae were reclassified as L. petauri [7]. Experimental studies have suggested that L. petauri may be more virulent than L. garvieae, raising the possibility that some human cases attributed to L. garvieae might actually have been L. petauri [2, 7]. As most human isolates were identified by 16S rRNA sequencing alone, genomic confirmation is needed to clarify the true burden of L. petauri infection. Notably, L. garvieae was recently implicated in platelet transfusion-associated bacteremia [8], underscoring the emerging clinical relevance of Lactococcus species. Our case represents the first fatal case of L. petauri infective endocarditis, demonstrating the critical role of WGS for accurate species discrimination and assessment of clinical significance in clinical microbiology [9, 10].

In conclusion, to our knowledge, we report the first human case of L. petauri bloodstream infection with concurrent infective endocarditis and spondylitis. This case highlights the limitations of routine methods, such as MALDI-TOF MS and 16S rRNA sequencing, in distinguishing L. garvieae from L. petauri and underscores the need for studies to define the pathogenic potential of L. petauri.

The clinical isolate draft genome sequence has been deposited in GenBank (accession No.: JAVTLH000000000.1).